1. Field of the Invention
The present invention relates a toner image transfer apparatus for transferring a toner image onto a recording sheet in an electrophotographic copying machine, a laser beam printer or the like, and more particularly to an improvement of a toner image transfer apparatus for detecting and/or inputting various factors influencing the image transfer to automatically set transfer parameters such as a transfer bias, etc. on the basis of the detection and/or input result.
2. Description of the Related Art
In a copying machine, a laser beam printer, etc. which use the electrophotographic system, a toner image corresponding to image information is formed on a photosensitive drum and then transferred onto a recording sheet, thereby forming a recording image on the recording sheet. For example, in the case of the laser beam printer, the surface of the photosensitive drum is first charged to predetermined background potential, and then exposed to a laser beam modulated on the basis of the image information to form an electrostatic latent image. Subsequently, the electrostatic latent image is developed with toner to form a toner image as a visible image, the toner image thus formed is recorded onto a recording sheet and then the recording sheet after the toner image is transferred thereto is heated and fixed by a fixer, thereby obtaining a recording image.
In order to transfer the toner image onto the recording sheet in the above recording image forming process have been known a method of directly transferring the toner image from the photosensitive drum to the recording sheet, a method of primarily transferring the toner image onto an intermediate transfer belt having an endless sheet shape and then secondarily transferring the toner image from the intermediate transfer belt to the recording sheet, etc. Of these methods, the transfer method using the intermediate transfer belt is optimum to form a full color image for which plural color toner images are required to be superposed on one another, and it is applied to a color copying machine or the like.
In all the transfer methods, transfer members such as a corona discharger, a bias roll, etc. are disposed so as to confront image carriers such as a photosensitive drum, an intermediate transfer medium, etc. through a recording sheet, and the charges having the opposite polarity to the charges of toner are applied to the back surface of the recording sheet by these transfer members, whereby a toner image is electrostatically transferred to the surface of the recording sheet. For example, when the charge polarity of toner is minus (-), a transfer bias of (+) is applied to the transfer members disposed at the back surface side of the recording sheet, and charges of (+) are applied to the back surface of the recording sheet.
Here, environmental factors such as temperature, humidity, etc. and the type of a recording sheet are considered as factors influencing the electrostatic transfer of the toner image as described above. This is because these influence factors cause variation of the resistance value of the recording sheet, and thus even when the same magnitude of transfer bias is applied, a sufficient transfer electric field cannot be produced between the image carriers and the recording sheet due to these influence factors, and thus the transfer efficiency is remarkably lowered. Further, when a bias roll is used as a transfer member, variation of the resistance value of the bias roll due to time lapse is also considered as a factor influencing the electrostatic transfer.
Therefore, the conventional toner image transfer apparatus is designed so that the type of a recording sheet onto which a toner image is transferred is input through a user interface such as an operation panel or the like by a user, and also so that an environment sensor for measuring temperature/humidity in the image forming apparatus is provided, and the magnitude of the optimum transfer bias based on the type of the recording sheet and the detected temperature/humidity is read out from a table created in advance, and then the toner image is electrostatically transferred to the recording sheet with the transfer bias thus determined. Further, in order to grasp the variation of the resistance value of the bias roll due to time lapse, the current value flowing in the bias roll when a predetermined voltage is applied to the bias roll is measured, and the transfer bias is determined in consideration of the magnitude of the current value.
However, even when the factors influencing the electrostatic transfer are detected and/or input and the transfer bias is automatically determined on the basis of these influence factors, a trouble may occur in the transfer of the image toner due to the type of the recording sheet used by the user. That is, even when the type of the recording sheet is indicated merely as "plain paper", the resistance value of the recording sheet is dispersed due to the difference of basis weight, the difference of the surface status or the like, and thus it is difficult to automatically set the transfer bias optimum to each recording sheet. Further, the table used to search the optimum transfer bias on the basis of the temperature/humidity and the type of the recording sheet cannot be extremely finely divided in view of the relationship with the cost effect, and thus the probability that the transfer bias voltage which is surely conformed with the detected temperature/humidity can be applied to the recording sheet is extremely small.